1. Field of the Invention
This invention relates to wireless communications and, more particularly, to a power control system for a cellular communications system which uses acknowledgments.
2. Description of Related Art
A conventional wireless cellular communications system comprises a number of cell sites or base stations, geographically distributed to support transmission and receipt of communication signals to and from cellular units, often referred to as mobile units or wireless units which may actually be stationary or fixed. Each cell site handles voice and/or data communications over a particular geographic region called a cell, and the overall coverage area for the cellular system is defined by the union of cells for all of the cell sites, where the coverage areas for nearby cell sites overlap to some degree to ensure (if possible) contiguous communications coverage within the outer boundaries of the system's coverage area.
In a wireless cellular communications system, a base station and a wireless unit communicate voice and/or data over a forward link and a reverse link, wherein the forward link carries communication signals from the base station to the wireless unit and the reverse link carries communication signals from the wireless unit to the base station. There are many different schemes for determining how wireless units and base stations communicate in a cellular communications system. For example, wireless communications links between the wireless units and the base stations can be defined according to different radio protocols, including TDMA (time-division multiple access), FDMA (frequency-division multiple access), and CDMA (code-division multiple access).
Traditional voice communications rely on communications links dedicated to the wireless unit. For example, an active wireless unit can have a dedicated forward link channel and a dedicated reverse link channel for the duration of a voice connection. Cellular communications systems based on traditional voice communications systems typically group voice and/or data information into traffic frames or radio packets defined by a certain time interval, for example 20 ms, and send the traffic frames over the dedicated communications link between the wireless unit and the base station. Certain wireless data communications systems take advantage of the bursty nature of data communications and use packet switched links to establish packet data connections for multiple wireless units sharing communications links over which radio packets are transmitted. In doing so, the wireless communications links are used more efficiently, thereby allowing increased capacity. In certain wireless data applications, a transmitting unit or station transmits radio packets to a receiving unit or station, and the transmitting unit or station waits to receive an acknowledgment signal from the receiving unit or station indicating that the receiving unit received the radio packet. If, after a period of time, the acknowledgment signal for the radio packet is not received by the transmitting unit or station, the transmitting unit or station re-transmits the radio packet.
Wireless communications systems use power control to improve system performance and increase system capacity by controlling the power levels transmitted by the wireless units and/or the base stations. Power control is generally done by the receiving unit or station measuring the signal strength from the transmitting station or unit. The receiving unit or station can adjust its transmit power based on the received signal strength, and/or the receiving unit or station can relay power control information to the transmitting unit which adjusts its transmit power level in response to the power control information. The power level transmitted by every wireless unit is typically under the control of the serving base station, and the base station performs power control to reduce the power level that each wireless unit is transmitting while maintaining a good quality reverse link. By decreasing the power level that each wireless unit is transmitting, system-wide interference created by the transmissions of the wireless units is reduced. Such a scenario allows increased capacity for the wireless cellular communications system because the overall signal to interference ratio increases for all wireless units in the wireless cellular communications system.
Power control is especially important in CDMA systems in which wireless units share the same frequency channel. If the power of each wireless unit is not controlled, stronger received signals can interfere with the weaker signals received at the base station on the same frequency channel, thereby decreasing the probability that the weaker signals will be received. Power control is provided by the base station for each wireless unit within the base station coverage area so that the signals from the wireless unit are received by the base station at the same level. As such, the nearby wireless unit does not overpower the far away wireless units.
Various power control techniques are used in CDMA wireless communications systems based on the well-known IS-95 standard. When attempting to gain access to the CDMA system, a wireless unit uses a random access procedure over an Access Channel. A power control system determines the power level to be used by the wireless unit in transmitting over the Access Channel. To determine access power, the wireless unit sends access messages over the Access Channel and receives (or fails to receive) acknowledgment signals for the access messages. After transmitting an access message at a specified power level, the wireless unit waits for an acknowledgment signal from the base station. If an acknowledgment signal is received, the sending of access messages ends and the power level of the acknowledged access message is used to set the transmit power of the wireless unit over the Access Channel. If no acknowledgment is received for the access message, the wireless unit transmits the access message at an increased power level until an acknowledgment signal is received.
On the reverse link, autonomous power control is used to establish the reverse link and react to large path loss fluctuations on the reverse link. Each wireless unit performs a signal strength measurement based on the power level of a pilot signal from the base station as well as the sum of all the base station signals received at the wireless unit. This signal strength measurement is used by the wireless unit to adjust the transmitter power of the wireless unit. The stronger the received signal, the lower the wireless unit sets its transmitter power because reception of a strong signal from the base station indicates that the wireless unit is either close or has a good path to the base station. As such, the wireless unit can use less transmitter power, thereby lowering the interference it produces for other wireless units.
Directed power control is also used on the reverse link where the base station sends power control bits to the wireless unit. In performing directed power control, the base station performs signal strength measurements, such as signal to noise ratio measurements, 16 times per 20 ms traffic frame. A traffic frame is a basic timing interval for transmitting radio frequency (RF) signals over the air. The base station sends to the wireless unit power control bits which instruct the wireless unit to increase or decrease power to approach a target signal strength measurement. The power control bits are sent 16 times per 20 ms frame, and each bit produces a 1 dB change in wireless unit power. For each consecutive frame, the target signal strength measurement is reduced a small amount, for example a value which can reduce the target signal strength measurement by 1 dB after 35 frames, until a frame error occurs. A frame error occurs when a frame (e.g., “bad frame”) contains erroneous bit(s), but depending on the number of erroneous bits and the implementation, erroneous bit(s) can be corrected. When a frame error does occur, the target signal strength measurement is increased a relatively large amount, for example about 3 dB, and the base station instructs the wireless unit with power control bits to approach the target signal strength measurement.
On the forward link, lowering the power level of the base station also reduces the system-wide interference, thereby improving the signal to noise ratio for wireless units being served by other base stations. The forward link power control strategy involves the wireless unit determining the forward link Frame Error Rate (FER) over a predetermined number of frames and reporting the frame error rate measurements back to the base station. Usually, a Frame Error Rate (FER) is associated with a wireless communication system where the FER is typically defined as the ratio of bad frames to the total number of frames transmitted through a communication link between the base station and the wireless unit. Based on the FER measurements for the forward link, the base station adjusts the forward link power. For example, when a base station transmits a forward traffic frame, the wireless unit determines whether the received forward traffic frame is bad or not by performing error detection, for example using a cyclic redundancy check (CRC). The wireless unit will indicate such a determination to the base station using the error indicator bit (EIB) in the next reverse traffic frame from the wireless unit to the base station. Upon receiving reverse traffic frames from the wireless unit, the base station examines the error indicator bits. If the EIBs indicate no frame errors, the power is decreased by d (for example, ¼ dB) every 80 frames, but if the EIB indicates a frame error, the power is increased by u (for example, 1 dB) until a frame error is not indicated by an EIB bit. In an alternative embodiment, a wireless unit can send a power message report that contains the number of errored frames received and the total number of frames received during the interval covered by the report. In the alternative embodiment, if no frame error is received, the power level is decreased by d every 56 frames, but if an unacceptable number of errored frames occur during a reporting interval, the power is increased by u.